This invention relates to insulators and more particularly to very low frequency (VLF) antenna base insulators.
VLF antennas frequently are very tall, narrow structures which rest upon a base insulator on one end and extend vertically well over a thousand feet into the air. They derive their lateral support from guy wires extending out radially and downward from several different heights along the vertical length. The weight of some of these structures has been estimated at about three million pounds. Some such antennas are operated electrically at about 250 KV (rms) at 30 KHz. The base insulator(s) therefore have tremendous mechanical and electrical load requirements, some of which are conflicting.
Some failures have been experienced with prior art type existing base insulators and, because of the costs involved in replacing them, the whole problem associated with acquiring insulators for VLF antennas, including design, manufacture and acceptance testing, must be re-examined.
Existing prior type insulators are made of porcelain cast in bell-shaped units which are then used in clusters of three, and stacked in three tiers. The size of the insulators is limited due to the difficulties associated with casting the material. Normal insulator technology makes use of sheds and petticoat rings which become extremely difficult to cast flawlessly as the size of the insulator increases. As a consequence, existing insulators have been cast smooth so that a large insulator could be cast flawlessly; the lack of sheds and petticoats, however, place a limit on the electrical performance of the insulator. This, in turn, has necessitated stacking insulators in three tiers in order to handle the electrical load which in turn has caused mechanical problems because of the increased length. While porcelain demonstrates a very high compressive strength, it is severely limited in its ability to withstand tensile forces and lateral forces. In addition, the problems associated with minute cracks and flaws, contaminants and humid conditions become compounded at VLF frequencies due to heating in the immediate vicinity of the flaw or contaminant. A detailed analysis of each of the problems associated with these prior type base insulators show that, in all cases, the critical area is the surface of the insulator. Not only does the maximum mechanical stress occur at the surface, but the electrical problems are most acute on the surface, i.e., contamination, field concentration, minute cracks which lead to corona, and flashover. It has been found that the whole philosophy of using only ceramics as the antenna base material needed be re-examined and a new design approach taken.